David Cameron has denied that the international community is behaving in a "patronising" way towards the government of Nigeria by sending experts to assist in the hunt for almost 300 abducted schoolgirls.

Mr Cameron stressed that his decision to send a small group of UK experts to offer advice and logistical support came in response to a request from Nigerian president Goodluck Jonathan.

Britain, the US, China and France have all now offered assistance, and former prime minister Gordon Brown has confirmed that air and satellite surveillance is to be extended to neighbouring countries Cameroon, Chad and Niger amid fears that the 276 girls kidnapped from a school in northern Nigeria three weeks ago may be trafficked across the border to be sold as slaves by captors from Islamist militant group Boko Haram.

Asked about suggestions in Nigeria that the offer of international help was patronising, Mr Cameron told BBC Radio 5 Live: "We want to work with them and to help, but we are certainly not patronising the Nigerian government.

"This is an extremely difficult situation. These girls have been kidnapped and taken into a jungle area three times the size of Wales. It is an incredibly difficult thing grappling with these extremist Islamist groups that are prepared to go to any lengths and do appalling things.

"That is why we have a long-standing arrangement with the Nigerian government and we have police advisers and military advisers who work with them long before this crisis started. We offered help right after the abduction was made and we are now stepping up what we are doing, but all the time working with and asking the Nigerian government what they want.

"I spoke directly with the president myself and made an offer which he accepted. But I wouldn't want to go beyond what it is they want us to do."

Mr Brown, who is the United Nations special envoy on global education, said that he had spoken last night to US secretary of state John Kerry, who agreed that air and satellite surveillance would be extended into neighbouring countries.

The former PM urged British people to sign the Bring Back Our Girls petition, which he said was putting pressure on the Nigerian government to take action and making clear that the international community shared the "revulsion" of ordinary Nigerians for Boko Haram.

Speaking from Nigerian capital Abuja, Mr Brown told BBC1's Breakfast: "The more people who can sign the petition the more I think the Nigerian government officials and others will want to take action."

Mr Brown has also been working with the Nigerian authorities on a safe schools initiative, being launched today.

He said: "The first thing is to locate the girls. It is every parent's nightmare that your daughter or son could be kidnapped and you never see them again, and particularly so as the danger is that they are being trafficked into other countries as sex slaves.

"I have been working with the UK and US governments and now China and France to get the resources for surveillance for locating these girls - satellite and also air coverage.

"I hope we can have a special push over the next few days to see if we can find the girls. But we have also got to make the schools safe for other girls and boys who are afraid of the terrorists."

Asked whether the authorities in Abuja were doing enough to find the girls, Mr Brown said: "I don't think you can have any doubt now about the urgency with which this is being addressed. I have been pressing for action for three weeks. We have got a combination of international resources and we have got a government which is prepared now to take action about the schools in the country.

"We are going to have an action plan within the next few days. But of course the first thing is to see can we get these girls, who have been so sadly removed from their homes, can we get them back? That's where the energy is in the next few days."

Mr Brown said that Boko Haram did not represent a significant part of the Nigerian people: "This is a small group of terrorists who are wreaking havoc... Once people realise the full strength of international and Nigerian opinion against these terrorists, I think they are going to be pushed back from now on.

"I think that there is revulsion within Nigeria. I think we will see that this international opinion is having a huge effect."

The team of UK experts has arrived in the Nigerian capital, Abuja, said the Foreign Office.

An FO spokesman said: "A team of UK experts who will advise and support the Nigerian authorities in their response to the abduction of over 200 schoolgirls touched down in Abuja, Nigeria, this morning.

"The team is drawn from across government, including the Department for International Development, Foreign and Commonwealth Office and Ministry of Defence, and will work with the Nigerian authorities leading on the abductions and terrorism in Nigeria.

"The team will be considering not just the recent incidents but also longer-term counter-terrorism solutions to prevent such attacks in the future and defeat Boko Haram.

"The team will be working closely with their US counterparts and others to co-ordinate efforts."

Mr Cameron told Sky News: "This is a ghastly situation. As I said, an act of pure evil. Straight afterwards, the Foreign Secretary rang his counterpart in Nigeria to offer help. That offer wasn't taken up.

"I rang President Jonathan in the last couple of days to make a further offer and we've sent out a team of experts to help. They are a team that are very much complementary to what the Americans have sent - experts in counter-terrorism, policing counter-insurgency, but also helpers on aid and education."